There is an ongoing need for portable radios, especially hand-portable radios, that allow a user to communicate with an airborne or satellite transceiver, where the communication includes information on the location of the user. While such radios have many uses, they are especially important in emergency and search and rescue situations.
Emergency and search and rescue radio location systems are known in the art. In one prior art arrangement known as the SARSAT System, an emergency radio transmitter continuously broadcasts a beacon signal at 406 MHz. The beacon signal is picked up and recorded by a SARSAT satellite moving in low earth orbit. Later on in its orbit, the SARSAT satellite passes over a command and control base station to which it downloads the recorded signals picked up from continuous transmitter.
The Doppler shift of the recorded signal as it approached, crossed over and receded from the beacon transmitter is analyzed in the base station and the zero Doppler time determined. By knowing the orbital location of the SARSAT satellite at the zero Doppler time, a line of position of the beacon transmitter is found. After several passes an approximate slant range and approximate position of the beacon transmitter on the earth's surface determined. This estimated position is then communicated to a rescue unit which then commences a local area search.
This approach has a number of weaknesses well known in the art. For example, the position of the emergency beacon transmitter is known only approximately, there is a substantial time lag between initiation of the emergency transmitter and the local position determination, and anyone can listen in so that the emergency signal and Doppler information are not secure.
In another prior art emergency rescue system, a mobile emergency radio sends a signal directly to a local rescue unit to allow the local rescue unit to home in on the position of the emergency radio. While such units are effective, they suffer from a number of limitations well known in the art, as for example, the rescue unit must be within range of the radio (typically line of sight) before any indication of the presence of radio can be obtained, the rescue unit must carry direction finding equipment in order to home in on the radio, and considerable time may be required to locate the emergency radio, thereby prolonging the exposure of the rescue unit and the user of the radio to any unfriendly personnel which may be in the area.
In a third prior art emergency rescue system, a standard marine radio transceiver is coupled to a LORAN or OMEGA or satellite geolocation receiver provided with a speech synthesizer. When an emergency switch on the system is activated, the normal transceiver functions are disabled and it transmits a synthesized speech "Mayday" voice call and synthesized voice stating the latitude and longitude, on the emergency channel. Other information about the vessel or emergency may also be rendered in voice by the speech synthesizer and transmitted at the same time. A provision is made so that the emergency transmitter can be disabled remotely by a signal sent to the unit. A difficulty with this system is that it has very limited range, uses open channels that can be monitored by anyone, announces the position of the emergency unit in plain language by voice, takes a substantial time to communicate the position of the emergency unit and can be disabled by a remote signal. This makes the users especially vulnerable to any unfriendlies that may be monitoring the emergency channel.
Thus, there continues to be a need for an improved radio suitable for communication with airborne or satellite transceivers or relays and which automatically provides accurate information on its local position and which is, preferably, protected against undesirable eavesdropping.